1. Field of the Invention
This invention relates generally to a pressure sensor and more particularly to a multi-channel electronically scanned pressure sensor for cryogenic environments.
2. Description of the Related Art
There are currently 17 cryogenic wind tunnels in operation worldwide, most operate in a temperature range of -190.degree. C. to +70.degree. C. The coldest temperature falls approximately 140.degree. C. below the lowest mil-spec temperature for the performance rating of electronic components. In an effort to develop higher lift and lower drag airfoils, aerodynamicists are striving to study boundary layer behavior at very high Reynolds numbers. Cryogenics is used to lower flow temperatures to achieve high Reynolds numbers by increasing flow density. These low temperatures present problems with instrumentation hardware designed to measure pressure when subjected to this cold environment.
The prior art uses adhesive bonding of individual silicon pressure sensing die of silicon or Pyrex. A stress isolation pedestal may also be incorporated. An adhesive material is required to secure each sensor to the substrate. Since the adhesive exhibits a large variation in coefficient of thermal expansion (CTE) with temperature, this results in large apparent strains being exerted upon the sensors as thermal cycling occurs. Adhesive materials with critical transition temperatures above 77 K will also cause thermally induced offset variations, which will cause non-repeatability of sensor output data in cryogenic applications. This problem is more accentuated with the use of thinner membrane thicknesses necessary for accurate measurements in cryogenic wind tunnels.